Step And Flash Imprint Lithography Research Articles

In situ characterization of enhanced thermal performance by periodic nanostructures on the surface of a microchannel

In situ formation of nanofins (i.e. isolated and dispersed precipitation of nanoparticles) on flow conduits induces changes to the surface morphology for heat exchange, increasing effective surface area and enhancing their cooling performance. Furthermore, the existence of these nanostructures has a significant influence on heat transfer, as opposed to the thermo-physical properties (e.g. thermal conductivity) of cooling fluids enhanced by addition of nanoparticles. This study conducted an in situ analysis exploring the effects of nanoscale surface modifications on forced convective heat transfer and thermal performance. A numerical heat transfer analysis, based on the conductive/convective heat transfer between a fin base and deionized water (DIW), was performed with the assumption that periodic nanostructures existed on the heated microchannel surface. Predictions of enhanced thermal performance (between 37% and 143%) from the resultant increase in the effective heat transfer area were validated for flows over these nanostructures (artificial nanofins) fabricated by SFIL (step and flash imprint lithography) inside a microchannel. Polydimethylsiloxane microchannels are bonded to a silicon wafer containing thin-film thermocouple arrays deposited onto artificial nanofins, which had been fabricated a priori for the in situ characterization of thermal performance. It was found that the convective heat transfer Nusselt number (Nu) increased from 61% to 110%. Additionally, a theoretical analysis of the pressure drop was also successfully achieved for a comprehensive understanding of the heat transfer characteristics at the fluid-wall interface of a microchannel.

Planarizing material for reverse-tone step and flash imprint lithography

Reverse-tone step and flash imprint lithography (S-FIL/R) requires materials that can be spin-coated onto patterned substrates with significant topography and that are highly planarizing. Ideally, these planarizing materials must contain silicon for etch selectivity, be UV or thermally curable, and have low viscosity and low volatility. One such unique material, in particular, a branched and functionalized siloxane (Si-12), is able to adequately satisfy the above requirements. This paper describes a study of the properties of epoxy functionalized Si-12 (epoxy-Si-12) as a planarizing layer. An efficient synthetic route to epoxy-Si-12 was successfully developed, which is suitable and scalable for an industrial process. Epoxy-Si-12 has a high silicon content (30.0%), low viscosity (29 cP at 25°C), and low vapor pressure (0.65 Torr at 25°C). A planarizing study was carried out using epoxy-Si-12 on trench patterned test substrates. The material showed excellent planarizing properties and met the calculated critical degree of planarization (critical DOP), which is a requirement for a successful etch process. An S-FIL/R process using epoxy-Si-12 was demonstrated using an Imprio® 100 (Molecular Imprints Inc., Austin, Texas) imprint tool. The results indicate that epoxy-Si-12 works very well as a planarizing layer for S-FIL/R.

Fast Graph Transformation based Direct Solver Algorithm for Regular Three Dimensional Grids

This paper presents a graph-transformation-based multifrontal direct solver with an optimization technique that allows for a significant decrease of time complexity in some multi-scale simulations of the Step and Flash Imprint Lithography (SFIL). The multi-scale simulation consists of a macro-scale linear e lasticity model with thermal expansion coefficient and a nano-scale molecular statics model. The algorithm is exemplified with a photopolimerization simulation that involves densification of a polymer inside a feature followed by shrinkage of the feature after removal of the template. The solver is optimized thanks to a mechanism of reusing sub -domains with similar geometries and similar material properties. The graph transformation formalism is used to describe the algorithm - such an approach helps automatically localize sub-domains that can be reused.

Large Area, Facile Oxide Nanofabrication via Step-and-Flash Imprint Lithography of Metal–Organic Hybrid Resins

Step-and-flash imprint lithography (S-FIL) is a wafer-scale, high-resolution nanoimprint technique capable of expansion of nanoscale patterns via serial patterning of imprint fields. While S-FIL patterning of organic resins is well known, patterning of metal-organic resins followed by calcination to form structured oxide films remains relatively unexplored. However, with calcination shrinkage, there is tremendous potential utility in easing accessibility of arbitrary nanostructures at 20 nm resolution and below. However, barriers to commercial adoption exist due to difficulties in formulating polymerizable oxide precursors with good dispensability, long shelf life, and resistance to auto-homopolymerization. Here we propose a solution to these issues in the form of a versatile resin formulation scheme that is applicable to a host of functional oxides (Al2O3, HfO2, TiO2, ZrO2, Ta2O5, and Nb2O5). This scheme utilizes a reaction of metal alkoxides with 2-(methacryloyloxy)ethyl acetoacetate (MAEAA), a polymerizable chelating agent. Formation of these inorganic coordination complexes enables remarkable resistance to auto-homopolymerization, greatly improving dispensability and shelf life, thus enabling full scale-up of this facile nanofabrication approach. Results include successively imprinted fields consisting of 100 nm linewidth gratings. Isothermal calcination of these structures resulted in corresponding shrinkage of 75-80% without loss of mechanical integrity or aspect ratio, resulting in 20 nm linewidth oxide nanostructures.

Reduction of pattern peeling in step-and-flash imprint lithography

A strong adhesion force between resist and underlayer materials was employed to reduce resist pattern peeling generated from de-molding in step-and-flash imprint lithography (SFIL), while simultaneously minimizing the adhesion force between resist material and the template surface with fluorinated surfactants. The reduction in resist pattern peeling by the interaction between the fluorinated SiO2 resist with epoxy groups and UV reactive underlayer materials with epoxy groups during UV irradiation was investigated. An optimized resist material formulation and high adhesion strength between resist and underlayer materials led to the well-patterned 80nm resist lines on an underlayer for 100 imprints. The underlayer material was modified by a thermal crosslinking reaction of the aminoplast at 130°C before nanoimprinting and, in conjunction with a cationic epoxy chemical reaction between the resist and the underlayer during UV irradiation before de-molding, proved to be quite effective for SFIL defect reduction.

Fabrication of Three-Dimensional MIS Nano-Capacitor Based on Nanoimprinted Single Crystal Silicon Nanowire Arrays

We report fabrication of single crystalline silicon nanowire based-three-dimensional MIS nano-capacitors for potential analog and mixed signal applications. The array of nanowires is patterned by Step and Flash Imprint Lithography (S-FIL). Deep silicon etching (DSE) is used to form the nanowires with high aspect ratio, increase the electrode area and thus significantly enhance the capacitance. High-! dielectric is deposited by highly conformal atomic layer deposition (ALD) Al2O3 over the Si nanowires, and sputtered metal TaN serves as the electrode. Electrical measurements of fabricated capacitors show the expected increase of capacitance with greater nanowire height and decreasing dielectric thickness, consistent with calculations. Leakage current and time-dependent dielectric breakdown (TDDB) are also measured and compared with planar MIS capacitors. In view of greater interest in 3D transistor architectures, such as FinFETs, 3D high density MIS capacitors offer an attractive device technology for analog and mixed signal applications.

Wafer-Scale, Highly-Ordered Silicon Nanowires Produced by Step-and-Flash Imprint Lithography and Metal-Assisted Chemical Etching

ABSTRACTA combinatory approach of Step-and-Flash Imprint Lithography (SFIL) and Metal-Assisted Chemical Etching (MacEtch) was used to generate near perfectly-ordered, high aspect ratio silicon nanowires (SiNWs) on 4" silicon wafers. The ordering and shapes of SiNWs depends only on the SFIL nanoimprinting mould used, thereby enabling arbitary SiNW patterns not possible with nanosphere and interference lithography (IL) to be generated. Very densely packed SiNWs with periodicity finer than that permitted by conventional photolithography can be produced. The height of SiNWs is, in turn, controlled by the etching duration. However, it was found that very high aspect ratio SiNWs tend to be bent during processing. Hexagonal arrays of SiNW with circular and hexagonal cross-sections of dimensions 200nm and less were produced using pillar and pore patterned SFIL moulds. In summary, this approach allows highlyordered SiNWs to be fabricated on a wafer-level basis suitable for semiconductor device manufacturing.

A Graph Grammar Model of the hp Adaptive Three Dimensional Finite Element Method. Part I

The first part of our paper presents a composite programmable graph grammar model for the self-adaptive two dimensional hp Finite Element Method algorithms (2D hp-FEM) with mixed triangular and rectangular finite elements. The two dimensional model is a starting point for the three dimensional model of self-adaptive hp-FEM presented in the second part of this paper. A computational mesh is represented by a composite graph. The operations performed over the mesh are expressed by the graph grammar rules. The three dimensional model is based on the extension of the two dimensional model with rectangular finite elements. In the second part of this paper, we conclude the presentation with numerical examples concerning the generation of the optimal mesh for simulation of the Step-and-Flash Imprint Lithography (SFIL).

A common gate thin film transistor on poly(ethylene naphthalate) foil using step-and-flash imprint lithography

In this paper the fabrication of flexible thin film transistors (TFTs) on poly(ethylene naphthalate) foil is reported, with the source–drain layer patterned by step-and-flash imprint lithography (SFIL) as a first step towards fully UV-imprinted TFTs. The semiconductor was deposited by inkjet printing of a blend of TIPS-pentacene/polystyrene. The bottom contact, bottom gate TFTs were fabricated with the foil reversibly glued to a carrier, enhancing the dimensional stability and flatness of the foil to result in a thinner and more homogeneously distributed residual layer thickness. The obtained performance of the TFT devices, showing a mobility of μ = 0.56 cm 2 V −1 s −1 with an on/off ratio of >10 7 and near-zero threshold voltage, was found to be in good agreement with similar, photolithographically patterned state-of-the-art devices recently reported in literature. The results presented here show the feasibility of SFIL as a roll-to-roll compatible and down scalable patterning technique on flexible PEN foil for the fabrication of bottom-gate, bottom-contact flexible high-quality TFTs.

Nanopatterning of PMMA on insulating surfaces with various anticharging schemes using 30 keV electron beam lithography

As a low cost and high throughput method for nanoscale pattern replication, step and flash imprint lithography (SFIL) with UV transparent masters is gaining prominence for its potential in photonics and integrated-circuit fabrication. However, dielectric materials appropriate for fabricating nanostructured SFIL masters present a challenge when employing electron beam lithography (EBL) because insulator substrates covered by polymeric resists such as PMMA tend to accumulate charge during EBL exposures, thereby degrading the process. In this work we explore the performance of four different EBL anticharging schemes for nanofabrication of dense arrays of dots having diameters 16–30 nm in PMMA on UV transparent fused silica (FS) substrates. These include overlayers of aluminum or a water-soluble conducting polymer, as well as sandwiching of Al or Cr thin films between the substrate and PMMA. The quality of patterns transferred from PMMA into the underlying metallic layers was analyzed, and the grain size of the metal was found to be the limiting factor determining the edge roughness. The best resolution was attained employing the conducting polymer top-coating. This scheme also involves fewer processing steps. The authors have used this technique for lift-off of Cr and Au as well as Cr masked etch transfer of nanosized patterns into glass substrates for UV-transparent master mold fabrication.

Nanostructured Polymer Brushes by UV‐Assisted Imprint Lithography and Surface‐Initiated Polymerization for Biological Functions

AbstractFunctional polymer brush nanostructures are obtained by combining step‐and‐flash imprint lithography (SFIL) with controlled, surface‐initiated polymerization (CSIP). Patterning is achieved at length scales such that the smallest elements have dimensions in the sub‐100 nm range. The patterns exhibit different shapes, including lines and pillars, over large surface areas. The platforms obtained are used to selectively immobilize functional biomacromolecules. Acrylate‐based polymer resist films patterned by SFIL are first used for the selective immobilization of ATRP silane‐based initiators, which are coupled to unprotected domains of silicon substrates. These selectively deposited initiators are then utilized in the controlled radical SIP of poly(ethylene glycol)methacrylates (PEGMA). Nanostructured brush surfaces are then obtained by removal of the resist material. The areas previously protected by the SFIL resist are passivated by inert, PEG‐based silane monolayers following resist removal. PEGMA brush nanostructures are finally functionalized with biotin units in order to provide selective attachment points for streptavidin proteins. Atomic force microscopy and fluorescence spectroscopy confirm the successful immobilization of streptavidin molecules on the polymer grafts. Finally, it is demontrated that this fabrication method allows the immobilization of a few tens of protein chains attached selectively to brush nanostructures, which are surrounded by nonfouling PEG‐functionalized areas.

Fabrication of TiO2 Memristive Arrays by Step and Flash Imprint Lithography

Identical patterns and characteristics of sub-100 nm TiO2-based memristive systems on 4 inch silicon substrates were demonstrated using Step and flash imprint lithography (SFIL). SFIL is a nanoimprint lithography technique that offers the advantagess of a high aspect-ratio, reliable nano-patterns, and a transparent stamp that can be used to facilitate overlay techniques. The overlay process from the alignment system in IMPRIO 100 was appropriate for the fabrication of nanoscale crossbar arrays in this study. High-density crossbar arrays that consisted of TiO2 resistive switching material that was sandwiched between Pt electrodes with a width of 80 nm and a half-pitch of 100 nm were in turn replicated through successive imprinting and etching processes. The use of the direct metal etching process enhanced the uniformity of the TiO2/Pt interface. The electrical property of the crossbar arrays showed the bipolar switching behavior that resulted in the application of the nonvolatile resistive memory.

Application of Hierarchical Chromosome Based Genetic Algorithm to the problem of finding optimal initial three dimensional meshes for the self adaptive hp-Finite Element Method

Abstract The paper presents an algorithm for finding the optimal initial three dimensional mesh for the self-adaptive hp Finite Element Method (hp-FEM) calculations. In our previous work we have proposed a method of selection of optimal initial two dimensional mesh. The problem of finding the optimal mesh was solved by means of hierarchical chromosome based genetic algorithm (HCBGA). In this paper the extension of the HCBGA to three dimensional problems is presented. The selection of the optimal initial mesh will optimize the convergence rate of the numerical error of the solution over the sequence of meshes generated by the self-adaptive hp-FEM. This is especially true in the case when material data are selected as result of some stochastic algorithm and it is not possible to design optimal initial mesh by hand. The hierarchical genetic algorithm is tested on the numerical simulations of three dimensional Step-and-Flash Imprint Lithography (SFIL). The SFIL is a modern patterning process utilizing the photopolymerization to replicate the topology of the substrate onto the template. The SFIL is modeled as linear elasticity with thermal expansion coefficient. It is shown that the convergence rate of the self-adaptive hp-FEM depends on the quality of the initial mesh provided by the genetic algorithm.

CONVERGENCE OF ITERATIVE SOLVERS FOR NON-LINEAR STEP-AND-FLASH IMPRINT LITHOGRAPHY SIMULATIONS

The paper presents the analysis of the iterative solvers utilized to solve the non-linear problemof Step-and-Flash Imprint Lithography (SFIL) a modern patterning process. The simulationsconsists in solving molecular statics problem for the polymer network, with quadratic potentials.The model distinguishes the strong interparticle interactions between particles forminga polymer network, and weak interactions between remaining particles. It also allows for largedeformations, which all together implies the non-linear model. To illustrate the convergenceof the iterative solvers, we present snapshots of the deformation of the sample being subjectto the iterative solution. We claim that the animation is an interesting way of illustratingthe convergence of the iterative solvers.

Photocurable Silsesquioxane-Based Formulations as Versatile Resins for Nanoimprint Lithography

Methacrylate octafunctionalized silsesquioxane (SSQMA) was shown to be an ideal material with high performance for ultraviolet (UV)-based nanoimprint lithography (NIL). The total viscosity of SSQMA-based formulations was adjusted to between 0.8 and 50 cP by incorporating low-viscosity acrylic additives, making the formulations suitable for UV-based NIL. The cured SSQMA-based formulations showed numerous desirable characteristics, including low volumetric shrinkage (4%), high Young's modulus (2.445-4.272 GPa), high resistance to oxygen plasma, high transparency to UV light, and high resistance to organic/aqueous media, as a functional imprint material for UV-based NIL and step-and-flash imprint lithography (SFIL). Using both techniques, the SSQMA-based formulations were easily transferred to relief structures with excellent imprint fidelity and minimal residual thickness. Formulations containing 50% SSQMA (wt %) were able to reproduce high-aspect-ratio nanostructures with aspect ratios as high as 4.5 using bilayer SFIL. Transparent rigiflex molds and hard replica molds with sub-50-nm size features were reproducibly duplicated by using UV-NIL with the SSQMA-based resin. Nanostructures with feature sizes down to 50 nm were successfully reproduced using these molds in both UV- and thermal-NIL processes. After repeating 20 imprinting cycles at relatively high temperature and pressure, no detectable collapse or contamination on the replica surface was observed. These properties of the SSQMA-based resins make them suitable as inexpensive and convenient components in all NIL processes that are based on physical contact.

Unified Modeling Language description of the object-oriented multi-scale adaptive finite element method for Step-and-Flash Imprint Lithography Simulations

In the first part of the paper we present the multi-scale simulation of the Step-and-Flash Imprint Lithography (SFIL), a modern patterning process. The simulation utilizes the hp adaptive Finite Element Method (hp-FEM) coupled with Molecular Statics (MS) model. Thus, we consider the multi-scale problem, with molecular statics applied in the areas of the mesh where the highest accuracy is required, and the continuous linear elasticity with thermal expansion coefficient applied in the remaining part of the domain. The degrees of freedom from macro-scale element's nodes located on the macro-scale side of the interface have been identified with particles from nano-scale elements located on the nano-scale side of the interface. In the second part of the paper we present Unified Modeling Language (UML) description of the resulting multi-scale application (hp-FEM coupled with MS). We investigated classical, procedural codes from the point of view of the object-oriented (O-O) programming paradigm. The discovered hierarchical structure of classes and algorithms makes the UML project as independent on the spatial dimension of the problem as possible. The O-O UML project was defined at an abstract level, independent on the programming language used.

Wafer-Scale Near-Perfect Ordered Porous Alumina on Substrates by Step and Flash Imprint Lithography

Nanoporous anodic aluminum oxide (AAO) has been widely used for the development of various functional nanostructures. So far, highly ordered AAO on substrates could only be prepared using a nanoindentation method via hard stamping and lithographic techniques that are not scalable to a wafer-scale. Here we report on a step and flash imprint lithography (SFIL)-based method to fabricate a near-perfect ordered AAO with square and hexagonal lattice configuration on silicon substrate over 4 in. wafer areas. SFIL was used to prepattern a polymer mask layer, and wet-etching process was employed to transfer the nanopatterns to aluminum (Al) films, thus creating ordered nanoindentation on the Al surface. The ordered nanoindentation guides the growth of nanochannels in the anodization step to create the ordered nanoporous structures. The proposed wafer-scale process is compatible with standard semiconductor fabrication and offers substantial advantages over conventional Al patterning methods in terms of patterning areas, throughput, process simplicity, and process robustness, allowing up to 10 000 imprints or pattern transfer to the Al films.

Step and Flash Imprint Lithography for Semiconductor High Volume Manufacturing

Step and Flash imprint lithography (SFIL) has reached a critical point in its development life cycle. Resolution and image fidelity have been well established, and progress is being made on both the process and template fronts. This simple technique has the potential to become the next generation lithography (NGL) choice for semiconductor manufacturing. Yet, despite the positive outlook, SFIL has very little industry support. Concerns about defectivity, throughput, overlay, template cost, infrastructure, and a host of other critical issues are legitimate. More important, however, is the severe shortage of publicly available data on SFIL for semiconductor applications, giving the false impression that it is still in its infancy. This paper discusses the current status of SFIL and shows where industry collaboration is needed to solve the critical issues and demonstrate that it is a viable NGL technology.

Mass fabrication of resistive random access crossbar arrays by step and flash imprintlithography

Step and flash imprint lithography (SFIL) is a promising method recently used for nextgeneration lithographic technology because it is a high-speed process that canbe carried out at room temperature and low pressures. Improvements made toSFIL enable the replication of crossbar patterns with a high resolution and thedevelopment of suitable materials and techniques to achieve high resolution capability. Inthis study, SFIL is used to fabricate high-density random access crossbar arraysbased on a NiO resistive switching system. The bottom and top electrodes aretransferred onto silicon wafers perpendicular to each electrode using the inductivelycoupled plasma reactive ion etching (ICP-RIE) technique. Direct metal etchingwithout a wet-based process minimizes damage to the electrode surface. TheI–V curves of individualactive cells (70 × 70 nm2) for crossbar arrays reveal the unipolar resistive switching (RS) behaviour of the fabricated device. A highoff/on resistanceratio (>104) and reproducible resistance switching characteristics for each active cell were found indifferent fields and for different wafers. The experimental data indicate that high-densitycrossbar arrays can be well replicated and that the electrical performance of these arrays isreliable.

Pinning at template feature edges for step and flash imprint lithography

We present a theory of the pinning of an air-liquid interface at the feature edge of a template during the imprint step of step and flash imprint lithography (SFIL). Pinning occurs when the pressure at the air-liquid interface reaches the pressure of the bulk liquid. At this condition, there is no pressure gradient or driving force to move the liquid and fill the feature. We examined several parameters, including residual layer thickness, wettability of the feature edge, applied force on the template, and the radius of curvature of the feature edge, to study their effect on pinning. An optimal SFIL process window and template modifications are proposed that minimize the pinning at the feature edge while still preventing any extrusion along the mesa (pattern containing area on the template) edge.